In conventional circuit breakers, the arc formed during a breaking operation is normally extinguished using compressed gas. The arc extinction or interruption performance is thereby mostly defined by the blow pressure and the physical properties of the medium, e.g. the dielectric strength, the heat capacity as a function of temperature, the electronegativity and the thermal conductivity. For large ratings, compressed sulphur hexafluoride (SF6) is generally used.
Typically, the arc interruption performance is improved by increasing the blow pressure of the gas using the self-blast or puffer principle. Although up to a certain rating the required interruption performance can be achieved, compressed-gas circuit breakers have intrinsic limitations that make it impossible to increase the performance without affecting product cost constraints.
Aiming at a reduction in the size, circuit breakers employing a liquefied gas, in particular SF6, as the interruption medium have been proposed, e.g. in U.S. Pat. No. 3,150,245. However, the design according to U.S. Pat. No. 3,150,245 has inter alia the drawback that given the low critical temperature of SF6 the respective storage vessel has to be designed for extremely high pressures.
In consideration of the drawbacks of this design, further circuit breaker using SF6 have been proposed in U.S. Pat. No. 4,288,668, U.S. Pat. No. 4,307,274 and U.S. Pat. No. 4,736,080.
All these circuit breakers have in common that a relatively sophisticated ejection device is required for building up a pressure that is high enough for the insulation liquid to be ejected with the required blow pressure. For example, U.S. Pat. No. 4,307,274 discloses an operator for pumping liquid SF6 and in this context mentions a typical pressure of 2′500 psi (about 170 bar).
It is clear that for these circuit breakers not only a complex pressure build-up mechanism is required, but that also the walls of the pre-injection chamber have to be designed in a manner to withstand such high pressures. Ultimately, this leads to a relatively large size and high cost of the circuit breakers.